Why did Egyptians stop using papyrus?

Because it was mean to be used in the way they did.

One of the most admirable deeds of the ancient Egyptians was their writing. The first Egyptian writing was a system of figures called hieroglyphics. By 3100 BC the Egyptians were already using this type of writing. They carved or painted the symbols on the walls of tombs and temples. Later they developed other simpler forms of writing. The Egyptians used these forms to write on papyrus, a material similar to paper.